The goal of this project is to examine at the molecular level the specificity of mutagenesis in mammalian cells. To do this, we are developing a system using cloned DNA sequences that can be shuttled into and out of the eucaryotic chromosome. The use of the cloned for thymidine kinase (TK) from Herpes Simplex Virus provides a defined target of known DNA sequence in which a broad range of mutagenic events, including nucleotide substitutions, frameshifts and small additions or deletions, can be examined. The specific aims of this project are to establish an experimental system in which this can be accomplished and to initiate an examination of the mutational specificity of a variety of potential environmental mutagens as well as the molecular nature of spontaneous mutagenesis in mammalian cells.